When a series capacitor bank in a high-voltage network is provided with an overvoltage protection device employing a zinc oxide (ZnO) voltage-dependent resistor (varistor), there is a need to trigger a parallel spark gap when the varistor has been utilized to its thermal limit value. To reduce the cost of the voltage-dependent resistor, it is dimensioned in such a way that it is only able to absorb a limited amount of energy. When the absorbed energy exceeds the level specified in advance by this dimensioning, the parallel spark gap is activated, which spark gap is then extinguished by external current interruption or by-passing.
Different trigger devices for a protective spark gap of the kind just described are known. They are based on electrical energy being stored, for example in a capacitor, which is charged via line current supply devices with battery back-up. See, for example, U.S. Pat. Nos. 3,670,175 and 4,174,529.
An object of the present invention is to provide a protective means of the kind referred to in which triggering of the protective spark gap does not require any separate energy supply.